A fuel synthesis device includes: a supplier to supply CO2 and H2 gasses; a fuel synthesis catalyst to chemically react the CO2 and H2 gasses to synthesize fuel; a gas-liquid separator to liquefy the fuel into liquid and separate the liquid from a gas containing unreacted CO2 and H2 gasses, and CH4 gas as a side product; a return path to return the separated gas to a point between the supplier and the fuel synthesis catalyst; a bypass path to bypass, and merge downstream of, the return path, and to include a CH4 separator to separate the CH4 and a CH4 oxidation catalyst to oxidize the CH4; and a switching valve to selectively switch between communication with the return path and communication with the bypass path, wherein whether the switching valve communicates with the return path or bypass path is controlled based on the density of CH4.